Electrically heatable honeycomb body extruded from ceramic material

ABSTRACT

A honeycomb body formed of ceramic material includes channels separated from one another by walls, a front or anterior end face, a rear or posterior end face and a lateral surface. A fluid can flow through the honeycomb body in a flow direction from the front or anterior end face to the rear or posterior end face. The honeycomb body is less electrically conductive in at least one first portion than in at least one second portion. The portions lie one after another in the flow direction. A method for producing such a honeycomb body includes extruding poorly electrically conductive ceramic material. The honeycomb body is treated after extrusion in at least one second portion so that the electrical conductivity in the second portion is increased, whereas it remains unchanged in at least one first portion. The portions lie one after another in the flow direction.

The present invention concerns the field of electrically heatable honeycomb bodies for application in gas purification devices, in particular for exhaust gas systems of internal combustion engines.

It is known to equip purification systems for gases with honeycomb bodies through which the gases can flow, said honeycomb bodies being coated with catalytically active material and/or being formed as filters. Here, it is also known to use electrically heatable honeycomb bodies either in order to reach more quickly a temperature necessary for catalytic conversion processes or desorption or in order to ignite accumulated soot particles and/or to accelerate the conversion thereof.

In principle there are two different types of honeycomb bodies, specifically on the one hand extruded honeycomb bodies and on the other hand honeycomb bodies constructed from metal sheet layers. Whereas metal honeycomb bodies are suitable in principle for electric heating on account of their electrical conductivity, this is not initially the case with extruded honeycomb bodies if these are produced from ceramic material, which is usually the case. However, it is possible to extrude honeycomb bodies also from electrically conductive material, wherein electrically conductive honeycomb bodies are then produced that can be divided by slits such that an electric current path with properties suitable for electric heating is produced. Such honeycomb bodies are described for example in EP 0 452 125 A2 or EP 0 661 097 A1.

A development of such designs, in which two different materials having different electrical conductivity are used for extrusion, is additionally disclosed in EP 0 637 360 B1. The honeycomb body described there has, externally, a different electrical conductivity than internally.

A feature common to all extruded electrically heatable honeycomb bodies is that only relatively thin discs are required in principle for typical applications and in turn are relatively unstable due to slits and the materials used, such that the installation in a housing and the combination with further honeycomb bodies requires a relatively high level of effort and the durability under fluctuating thermal loads and mechanical loads is limited.

One object of the present invention is to solve the problems discussed in relation to the prior art, at least in part. A further object in particular is also to create an extruded honeycomb body that is electrically heatable in a portion, can be produced easily and is also mechanically stable.

In order to solve these problems a honeycomb body according to the features of claim 1 and a method for producing such a honeycomb body according to the features of claim 9 are used. Preferred exemplary embodiments that can be implemented individually or in technically feasible combinations with one another are specified in the dependent claims. The description, in particular in conjunction with the figures, explains the invention and specifies further exemplary embodiments.

The honeycomb body, extruded from ceramic material, has channels separated from one another by walls, an anterior end face, a posterior end face and a lateral surface, wherein a fluid can flow through the honeycomb body in a flow direction from the anterior end face to the posterior end face. Here, the honeycomb body is less electrically conductive in at least one first portion than in at least one second portion, wherein the portions lie one after another in the flow direction.

With regard to the material of the honeycomb body it is noted that this does not necessarily have to be formed completely with ceramic material, but other materials may also be present here (in a small proportion) where appropriate. For example, these can connect together segments of the honeycomb body that provide electrical conductivity and/or form lines/sensors. The proportion of the non-ceramic is generally so small here that the honeycomb body can be produced (in segments) by means of an extrusion method.

The channels (at least for the majority) usually run in a straight line between the anterior end face and the posterior end face. The channels can also be closed partially and/or in both directions at the end face with a closing member where appropriate when a fluid flow over the walls is to be enforced.

The honeycomb body or material thereof is particularly formed such that the exhaust gas (fluid) of a combustion process can be guided through there (permanently).

A first portion of the honeycomb body preferably directly adjoins the posterior end face. It is also preferable for the second portion to directly adjoin the anterior end face. The first portion and the second portion are interconnected, that is to say in particular are formed in a manner bordering on one another. The honeycomb body (except for coating, electrical connections, sensors, etc.) is even more preferably formed in one piece.

This arrangement of the portions with different electrical conductivity one after the other in the flow direction in particular makes it possible to provide a relatively short second portion in the flow direction that would not be mechanically stable as an individual disc, but in conjunction with the first portion is just as stable as a conventional (one-piece) ceramic honeycomb body. Here, the electrical conductivity in the second portion does not have to be homogeneously distributed, but can also be different in this portion, for example radially.

At least one electrically conductive current path interconnecting at least two contact regions is preferably defined in the second portion, wherein the contact regions are arranged on an end face and/or the lateral surface of the honeycomb body.

A current path is understood in particular to be a specific and/or directed and/or delimited course of the current through the walls of the second portion. Here, the current path extends starting from a connection contact (for example a voltage source) to a predefined other connection contact (for example an electric ground). The course is characterized in particular by at least one loop, a winding, a meander, a spiral or the like in the cross section of the honeycomb body in the second portion. The current path can be formed in particular by electrically insulating regions that extend in the cross section of the honeycomb body.

All configurations known in the prior art as used previously in (a plurality of) disc-shaped heatable honeycomb bodies can thus be realized. However, the structural tolerance for complicated current paths or a plurality of current paths connected in parallel is greater because it is practically no longer necessary to take into consideration the mechanical stability of the second portion. This second portion is connected virtually monolithically to the first portion, such that even a greater number of slits or very narrow current paths do not lead to mechanical problems.

The first portion does not influence the current distribution due to its lower electrical conductivity. Generally, the material of the first portion will essentially have the properties of an electrical insulator.

The mechanical stability of the (entire) honeycomb body makes it possible to provide a plurality of electrically conductive current paths in the second portion that preferably are each formed with the same electrical resistance. In this way, a large cross-sectional area can be heated very uniformly, which in known designs cannot be ensured in a mechanically stable manner in the case of long current paths of complicated form that extend for example in a meandering manner over a cross-sectional area. On the other hand it is also possible to provide current paths having different electrical resistance in order to heat different cross-sectional areas differently.

The electrically conductive portion and therefore all conductive current paths are particularly preferably arranged in the region of one of the end faces of the honeycomb body, in particular in the region of the anterior end face. This also means in particular that the second portion borders the (anterior) end face or includes said end face. In other words, this may also mean that current paths are provided only in this (second) portion and not in the other (first portion). In typical arrangements for purifying exhaust gases of internal combustion engines, in particular in motor vehicles, this is the design used most often because the regions of the honeycomb body lying further on in the flow direction can be heated most easily by a heatable anterior end face.

The second portion preferably has a radially and/or axially inhomogeneous electrical conductivity. A “radial” inhomogeneity is then present in particular when the second portion, considered starting from an axial center to the lateral surface, does not have constant electrical conductivity and/or a plurality of straight connection lines from the center to the lateral surface have an electrical conductivity that is constant, but differs from one connection line to another. An “axial” inhomogeneity is then present in particular when the second portion, considered starting from an end face toward a parallel delimitation face (considered in the direction of the axis), does not have constant electrical conductivity and/or a plurality of straight connection lines from the end face toward the delimitation face have an electrical conductivity that is constant, but differs from one connection line to another. The inhomogeneity is characterized in particular by a significant deviation, for example at least 20%, 40% or even 70%.

A honeycomb body according to the invention is preferably initially produced throughout from an electrically hardly conductive ceramic material, wherein the electrical properties of the electrically better conductive portion are then changed by adding electrically conductive material. Here, there are various possibilities for increasing the electrical conductivity in a portion. Since ceramic material, in particular extruded ceramic material, for honeycomb bodies is relatively porous, metal material can be immersed, for example by immersion in a melt or solution containing dissolved metals, whereby metal material is deposited in the immersed regions of the honeycomb body. Vapor deposition methods, sputtering methods or the end-face imprinting of metal material is also possible.

The second portion is particularly preferably formed as a raised region protruding on one of the end faces. The/all electrically conductive current paths can particularly preferably be formed as raised parts protruding from the end face or as raised current paths on an end face of the honeycomb body.

The second portion is then provided for example with a relief-like structure, in which the structures protruding as mountains form the electrically conductive current path, whereas the valleys or slits serve for electrical insulation. Depending on the selected method for production, either the electrical conductivity in the second portion can be increased first and then the structure can be carved out, or vice versa.

In principle it is possible in accordance with the invention to form two or more electrically conductive portions, for example on both end faces of a honeycomb body, but it is preferable to arrange all current paths as raised current paths only on one end face of the honeycomb body.

The present invention also relates to a method for producing a honeycomb body from ceramic material having channels separated from one another by walls, an anterior end face, a posterior end face and a lateral surface, through which honeycomb body a fluid can flow in a flow direction from the anterior end face to the posterior end face, wherein the honeycomb body is produced by extrusion of poorly electrically conductive ceramic material.

Here, poorly electrically conductive material is typically understood to be ceramic material, such as cordierite, which can be considered practically as an electrical insulator. In accordance with the invention an extruded honeycomb body is left untreated in at least one first portion following the extrusion, whereas it is treated in a second portion such that the electrical conductivity in this second portion is increased, wherein the portions lie one after another in the flow direction. An increase of the electrical conductivity is understood in this context to mean that the second portion is to demonstrate good electrical conductivity, which in particular means that the conductivity of the walls in this portion reaches values similar to those with honeycomb bodies produced from metal sheets or sintered metal.

This can be achieved in accordance with the invention preferably by treating the second portion by means of an immersion method, in particular by immersing this portion in a melt or solution of a metal or of a metal alloy. The porosity provided following the extrusion leads to an absorption of metal from a melt or solution, whereby the electrical conductivity can be significantly increased. Here, it is dependent on the selected method as to whether the immersion method is more advantageous before and/or after a potential firing process of the extruded honeycomb body.

The honeycomb body is preferably immersed in an immersion bath up to a predefined height in order to determine the height, that is to say the axial extent, of the second portion and to treat this portion uniformly. The duration of the immersion process, the temperature and/or the concentration of the immersion bath are selected here such that the desired electrical conductivity is set. The height of the immersion or the axial extent of the second portion is selected in particular to be much smaller than the axial length of the honeycomb body on the whole and for example is at most 30% or even only at most 10% thereof. The axial extent of the second portion thus can also be just a few millimeters [mm], such as at most 20 mm or even only at most 10 mm.

Alternatively, in accordance with another embodiment of the invention, the electrical conductivity of the second portion of the honeycomb body can be increased by a vapor deposition method or a spraying method for application of an electric material. It is particularly advantageous here if the method is performed prior to the firing of an extruded ceramic honeycomb body because a uniform penetration of the applied metal layer is then achieved.

If expedient, the above method steps can also be used in combination with one another (at different points of the production and/or with different materials where appropriate) in order to set the electrical conductivity.

The method can be developed such that the second portion is treated selectively by protecting predetermined separation regions of the honeycomb body by a mask or a protective coating (or the like) prior to the treatment, in particular separation regions that divide the second portion into at least one electrically conductive current path.

A homogeneous electrically conductive second portion can have, in some circumstances, a resistance that is unsuitable for electric heating, specifically an electrical resistance that is too low. In order to nevertheless achieve uniform heating of the entire region, the invention provides various possibilities for dividing the second portion electrically by means of separation regions such that at least one current path, preferably a plurality of current paths with suitable electrical resistance is/are produced. The separation regions are ensured in accordance with one exemplary embodiment of the invention by preventing the increase of the electrical conductivity in these regions, which is achieved for example by a mask on an end face of the honeycomb body or by a protective coating.

In another embodiment material is removed, preferably by milling, at one or both end faces prior to or following the treatment for increasing the electrical conductivity, such that at least one raised electrically conductive current path is formed or is produced after the treatment. Due to the milling or other material-removing methods, slitted or also larger separation regions, which act in an electrically insulating manner, are produced. In this way, designs can be produced in which the second portion demonstrating good electrical conductivity has structures similar to heatable honeycomb bodies produced from metal sheets or extruded from metal material. In particular, a winding or meandering current path can thus be produced easily. The knowledge regarding current distribution and contacting provided extensively in the prior art can be applied equally to the electrically conductive portions produced in accordance with the invention.

By way of example the following method steps are outlined:

-   -   A. Production of a honeycomb body from ceramic material having         channels separated from one another by walls, an anterior (in         particular flat) end face, a posterior (in particular flat) end         face and a (particularly cylindrical) lateral surface, through         which honeycomb body a fluid can flow in a flow direction from         the anterior end face to the posterior end face, wherein         -   a. the honeycomb body is produced in one piece, preferably             by means of extrusion,         -   and/or         -   b. the honeycomb body is produced preferably from segments             of extruded material joined together,         -   and/or         -   c. the honeycomb body is preferably provided (only) with             cordierite, such that a honeycomb body is produced using             electrically poorly conductive ceramic material.     -   B. Treatment of the honeycomb body, after step A., wherein         portions lying one after another in the flow direction are         created, after which the electrical conductivity in at least one         first portion remains (practically) unchanged and the honeycomb         body is treated in at least one second portion such that the         electrical conductivity in this second portion is increased,         wherein         -   a. raised regions are preferably formed on an (individual)             end face,             -   in particular by milling,         -   and/or         -   b. the portion of the honeycomb body that is to form the             second portion is provided with a metal,             -   preferably by means of an immersion bath in a melt                 and/or a solution,             -   and/or             -   preferably by means of a vapor deposition method and/or                 a spraying method,         -   and/or         -   c. a selective treatment is preferably performed, wherein             pre-determined separation regions of the honeycomb body are             protected by a mask or a protective coating prior to the             treatment.     -   C. Thermal treatment of the honeycomb body, in particular to         cure the material or the walls of the honeycomb body (firing).     -   D. Coating of the honeycomb body with a catalytically active         coating (optional).

Here, step B. (or the processes outlined therein) can be performed before and/or after step C.

Further details and exemplary embodiments of the invention, to which the invention is not limited however, will be explained hereinafter in greater detail on the basis of the drawing: here, details of the individual exemplary embodiments can also be combined with one another in a manner different from that selected here:

FIG. 1: schematically shows the structure of a ceramic honeycomb body with a first poorly electrically conductive portion and a second portion demonstrating good electrical conductivity,

FIG. 2: shows a honeycomb body according to the invention with an electrically conductive portion divided by slits,

FIG. 3: schematically shows a honeycomb body as in FIG. 2 with illustration of an associated electric circuit,

FIG. 4: shows an end-face view of a honeycomb body according to the invention, in which a plurality of parallel current paths are formed by separation regions, and

FIG. 5: schematically shows an illustration of an immersion method according to the invention.

FIG. 1 shows the principle structure of an extruded ceramic honeycomb body 1 according to the invention with an anterior end face 2 and a posterior end face 3, wherein the honeycomb body 1 has walls 10 which form channels 11. A fluid, in particular a gas, can flow through these channels 11 in the flow direction S. The honeycomb body 1 has an axial length L in the flow direction S and externally has a surrounding lateral surface 4. In a first portion 5 the honeycomb body is poorly electrically conductive, and in particular is electrically insulating for practical purposes. In a second portion 6, which borders the end face 2 in the present exemplary embodiment, the honeycomb body has good electrical conductivity similar to that of metal honeycomb bodies. In FIG. 1 it is also indicated that there may be not just one portion demonstrating good electrical conductivity, but a plurality of such portions. In this exemplary embodiment a further second portion 7 demonstrating good electrical conductivity is also formed in a manner bordering on the posterior end face.

FIG. 2 illustrates how an electrically conductive second portion 6 of a honeycomb body 1 can be electrically divided by separation regions, in the present case slits 8, in order to attain a desired electrical resistance. The method of dividing an electrically conductive disc by slits is well known in the prior art, and the measures and advantages known from this prior art can also be applied in the case of the present invention. Here, it is very important that only little attention has to be paid to the mechanical stability of the portion 6 demonstrating good electrical conductivity when forming slits 8 because this second portion 6 is connected integrally to the first portion 5 and obtains its stability from the first portion 5. However, it is important that the second portion 6 demonstrating good electrical conductivity has a height H that is smaller than a depth T of the slits 8. The slits 8 should reach as far as a boundary 9 within the first portion 5 in order to avoid short-circuits between the individual parts of the slitted second portion 6.

FIG. 3 illustrates how a honeycomb body according to the invention according to FIG. 2 can be incorporated in an electric circuit. In the present exemplary embodiment the second portion 6 is contacted on two sides of the lateral surface 4, specifically with a first connection contact 13 and a second connection contact 14. These can be connected to an electric current source 16 via electrical feed lines 15, where appropriate via a switch. A current path 12 is then formed in the second portion 6 of the honeycomb body 1 and in the present exemplary embodiment runs in a meandering manner around the slits 8 in the region of the end face 2. The second portion 6 can thus perform the same tasks for electrical heating as disc-shaped electrically conductive honeycomb bodies known from the prior art, but is an integral component of a larger honeycomb body and is thus mounted in a mechanically stable manner.

FIG. 4 schematically shows an end-face view of a honeycomb body 1 according to the invention, in which a division of the second portion 6 into a plurality of current paths 20, 21, 22, 23, 24 has been performed. The separation regions 19 can be either regions or slits made electrically non-conductive. In the present exemplary embodiment a relatively wide and short central current path 20 is provided, through which a relatively strong current can flow from the first connection contact 13 to the second connection contact 14 when a voltage is applied thereto. In honeycomb bodies through which fluids can flow the greatest heat output is often required in the center, which is why this embodiment may be particularly expedient.

Two decentral current paths 21, 22 are slightly longer and slightly less wide, such that lower partial currents can flow there through when a voltage is applied to the first connection contact 13 and the second connection contact 14. Even smaller currents flow in this case through the two lateral current paths 23, 24. FIG. 4 is only one example for the many possibilities provided to a person skilled in the art for the division of the second portion, in particular when a person skilled in the art only has to pay little attention to the stability of the structures produced because said stability is practically always ensured by the first portion 5.

It is noted that the arrangement of a portion demonstrating good electrical conductivity is indeed particularly preferred at one of the end faces, in particular the anterior end face 2 of a honeycomb body 1, however an arrangement at the posterior end face 3 or at both ends faces 2, 3 is also possible. Depending on the selected method for increasing the electrical conductivity in a portion, it is even also possible to produce such a portion in the interior of a honeycomb body and even to structure said portion where appropriate. If a protective coating is applied from both end faces, for example by immersion, that can also achieve separation regions in the entire honeycomb body via a suitable mask, precisely that portion in the interior of the honeycomb body not provided with a protective coating can thus be made electrically conductive by subsequent immersion of the honeycomb body in an immersion bath containing metal material. The invention therefore offers numerous possibilities for application, besides the described exemplary embodiments, for producing stable honeycomb bodies having one or more electrically heatable portions.

FIG. 5 purely schematically illustrates the coating process according to the invention of a honeycomb body 1. This honeycomb body is immersed in an immersion bath 18 in a container 17 to such a depth that a first portion 5 remains above the immersion bath 18 whereas a second portion 6 fills with liquid of the immersion bath 18 up to a height H and absorbs said liquid in part. Following the removal of the honeycomb body 1 from the immersion bath and following a further treatment where appropriate, in particular firing, a honeycomb body according to the invention having two portions 5, 6 of different electrical conductivity is thus produced.

By way of precaution it is also noted that the combinations of technical features shown in the figures are not generally compulsory. Technical features of one figure may thus be combined with other technical features of a further figure and/or the general description. This shall only not apply when the combination of features has been disclosed here explicitly and/or a person skilled in the art identifies that otherwise the basic functions of the device can no longer be fulfilled.

The preferred field of application of the invention is exhaust gas purification of internal combustion engines, in particular in motor vehicles. Exhaust gas purification systems generally contain not only an electrically heatable disc, but further honeycomb bodies as particle filter and/or as catalytic converter with a catalytically active coating. The present invention can therefore provide two components at the same time, specifically an electrically heatable disc and an integrally associated honeycomb body of which the dimensions, in particular the axial length thereof, can be selected independently of the axial length of the heatable disc.

LIST OF REFERENCE SIGNS

-   1 honeycomb body -   2 anterior end face -   3 posterior end face -   4 lateral surface -   5 first portion -   6 second portion -   7 further second portion -   8 slit -   9 boundary -   10 wall -   11 channel -   12 current path -   13 first connection contact -   14 second connection contact -   15 electrical feed lines -   16 current source -   17 immersion container -   18 immersion bath -   19 separation region -   20 central current path -   21 decentral current path -   22 decentral current path -   23 lateral current path -   24 lateral current path     -   H height     -   L axial length     -   T depth     -   S flow direction 

1-15. (canceled)
 16. A honeycomb body, comprising: a front end face, a rear end face and a lateral surface; channels being separated from one another by walls of extruded ceramic material for conducting a fluid through the honeycomb body in a flow direction from said front end face to said rear end face; at least one first portion and at least one second portion lying one after another in said flow direction; and the honeycomb body being less electrically conductive in said at least one first portion than in said at least one second portion.
 17. The honeycomb body according to claim 16, which further comprises: at least two connection contacts disposed at least on one of said end faces or on said lateral surface; said at least one second portion defining at least one electrically conductive current path interconnecting said at least two connection contacts.
 18. The honeycomb body according to claim 17, wherein said at least one electrically conductive current path includes a plurality of electrically conductive current paths.
 19. The honeycomb body according to claim 18, wherein all of said electrically conductive current paths are disposed in vicinity of one of said end faces.
 20. The honeycomb body according to claim 16, wherein said at least one second portion has at least one radially or axially inhomogeneous electrical conductivity.
 21. The honeycomb body according to claim 16, wherein the honeycomb body is better electrically conductive in said at least one second portion than in said at least one first portion, and said better electrically conductive at least one second portion includes electrically conductive material added to a finished pre-formed honeycomb body formed of the same material throughout.
 22. The honeycomb body according to claim 16, wherein said at least one second portion is formed as a raised region protruding on one of said end faces.
 23. The honeycomb body according to claim 18, wherein all of said current paths are formed as raised current paths on one of said end faces.
 24. A method for producing a honeycomb body, the method comprising the following steps: extruding poorly electrically conductive ceramic material to produce a honeycomb body having a front end face, a rear end face, a lateral surface and channels separated from one another by walls for conducting a fluid through the honeycomb body in a flow direction from the front end face to the rear end face; providing the honeycomb body with at least one first portion and at least one second portion lying one after another in the flow direction; treating the at least one second portion following the extrusion to increase electrical conductivity in the at least one second portion; and maintaining electrical conductivity unchanged in the at least one first portion.
 25. The method according to claim 24, which further comprises performing the treating step by using an immersion method.
 26. The method according to claim 25, which further comprises further performing the treating step by immersing one of the end faces of the honeycomb body in an immersion bath up to a predefined height.
 27. The method according to claim 24, which further comprises performing the step of increasing the electrical conductivity of the at least one second portion of the honeycomb body by a vapor deposition method or a spraying method.
 28. The method according to claim 24, which further comprises selectively treating the at least one second portion by protecting predetermined portions of the honeycomb body by a mask or a protective coating prior to the treating step.
 29. The method according to claim 24, which further comprises removing material on one or both of the end faces prior to or following the treating step to form or produce at least one raised electrically conductive current path following the treating step.
 30. The method according to claim 24, which further comprises performing the treating step prior to or following a firing of the extruded honeycomb body, and removing material at the end faces prior to or following the firing. 